This invention relates to a multireflector fiberoptic filter apparatus and method. In particular, the invention relates to a multireflector fiberoptic filter apparatus and method, wherein the transmittance and reflectance spectra are periodic in frequency, including an etalon with N equally spaced lossless reflectors with a specified period spacing between them.
The Fabry-Perot interferometer (FPI), sometimes called the Fabry-Perot etalon, consists of two mirrors separated by a distance L. Since its invention about 100 years ago, the bulk-optics version of the FPI has been widely used for high-resolution spectroscopy. In the early 1980""s, fiber optic FPIs consisting of mirrored fiber ends separated by an air gap were introduced as a filter technology for optical communications.
The simple, compact fiber FPI filters have proven very useful in lightwave communications, but suffer from a fundamental drawback: the gradual (6 dB/octave) dropoff of transmittance with optical frequency away from the transmittance peak. A filter with a flatter in-band response and greater out-of-band rejection would make it possible to increase the channel density and decrease the filter-related power penalty in dense wavelength division multiplexing (WDM) systems.
WDM is widely used in fiber optic communication to increase the data capacity of an optical fiber. Currently, 16, 32, or more data channels are transmitted in parallel on a single mode fiber using different optical carrier frequencies for each channel. To combine and separate these channels, a variety of frequency-selective components have been developed, including multilayer dielectric coatings, fiber Bragg gratings, arrayed waveguide gratings, and Mach-Zehnder chains. None of these techniques satisfies industry requirements for low cost, high-speed tunability, low optical insertion loss, and small size.
Thus, there is a need in the art for providing an apparatus and method for a fiber optic filter that is inexpensive to produce and that has high-speed tunablity, low optical insertion loss and is small in size.
Accordingly, the multireflector fiberoptic filter apparatus and method of the present invention includes an etalon with N equally spaced reflectors, wherein the transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (xcex94xcexd)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the medium between the reflectors; L=the separation between reflectors; and N is an integer=3,4,5, . . . .
In other aspects of the invention, the etalon is comprised of a serial arrangement of single mode optical fibers of a common length L, each with a reflector deposited on one end. In other aspects of the invention, the single mode optical fibers are aligned in tubes or in V-grooves formed on a silicon substrate, and are interfaced to one another end-to-end by means of one from a group including optical cement, epoxy, fusion splicing, or refractive-index-matching liquid.
In another aspect of the invention, the reflectors are multilayer quarterwave stacks comprising alternating layers of high and low index dielectric materials. In another aspect of the invention, the dielectric materials are chosen from a group including TiO2 and SiO2.
In yet another aspect of the invention, the reflectors are mirrors. In a further aspect of the invention, an optical circulator is connected to the etalon and an optical fiber is connected to the optical circulator for reflected output. In another aspect of the invention, a plurality of etalons are connected in series. In another aspect of the invention, a strain inducing device is attached to the etalon. In a further aspect of the invention, the strain inducing device is a piezoelectric element attached along the length of the etalon. In another aspect of the invention, delay sections are added between etalons.
In another embodiment of the invention, a fiber optic system contains a first input fiber, a second transmitted output fiber and a third reflected output fiber, connected to a multireflector etalon for separating equally spaced frequency channels into two groupsxe2x80x94one transmitted and the other reflected. In said system, the transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (xcex94xcexd)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the optical fiber medium between the reflectors; and L=the separation between reflectors. Said etalon is comprised of the serial arrangement of single mode fibers of a common length L separated by N reflectors, N=3,4,5, . . . , and is connected to the second transmitted output fiber. The transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (xcex94xcexd)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the medium between the reflectors. An optical circulator is connected to the first input fiber, the etalon, and the third reflected output fiber.
In another aspect of the invention, a plurality of etalons is provided in series. In a further aspect of the invention, fiber delay sections are added between the etalons.
In another embodiment of the invention, in a single mode optical fiber in which optical frequency channels are equally spaced, a method for wavelength division multiplexing is provided where a subgroup of the equally spaced frequency channels are selected for transmission and the remainder are reflected wherein the transmittance and reflectance spectra are periodic in frequency, the method including the step of providing an etalon with N equally spaced reflectors. The transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (xcex94xcexd)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the optical fiber medium between the reflectors. A single mode optical fiber is connected to the etalon. The etalon is connected to an output transmission fiber and equally spaced optical frequency channels are transmitted from the optical fiber to the etalon.
In a further aspect of the method, an optical circulator is connected between the single mode optical fiber and the etalon and a reflected output channel is connected to the optical circulator. In another aspect of the method, a tuning device is added to the etalon. In a further aspect, a plurality of etalons are provided and fiber delay sections are included between the etalons.